The present invention relates to a powder spraycoating control system and its combination with a powder feed apparatus or with powder spraycoating equipment.
The present invention relates as well to such powder feed apparatus and powder spraycoating equipment that comprise an injector serving as the powder pump as well as such that comprise a dense phase powder pump.
Dense phase powder pumps comprise at least one feed chamber fitted with a powder intake valve and a powder outlet valve. The feed chamber is alternatingly connected to a vacuum source during a suction stage and to a source of conveying compressed air during a discharge stage. The vacuum from said vacuum source aspirates powder through the open powder intake valve into the feed chamber while the powder outlet valve is closed. The conveying compressed air from the source of conveying compressed air discharges powder from within the feed chamber through the open outlet valve while the intake valve is closed. Most dense phase powder pumps comprise two feed chambers operating in mutually time-staggered manner in order that alternatingly coating powder shall be aspirated each time into one feed chamber while the pertinent other feed chamber is discharging coating powder.
Different kinds of coating powder feed apparatus containing a dense phase powder pump are known for instance from the following documents which are incorporated by reference herein: JP 09/071,325 A, DE 196 11 533 B4, US 2000/0193704 A1 (=EP 1 644 131 A2), U.S. Pat. No. 7,150,585 B2 (=WO 2004/087331 A1) and US 2005/0178325 A1 (=EP 1 566 352 A2). A vacuum intake of at least one of the two feed chambers and in some embodiment modes also the compressed air intake of the feed chamber is/are fitted with a filter permeable to air but not to coating powder. The preferred filter material is a sintered one. Predominantly the powder intake and outlet valves are pinch valves.
The quantity of powder per unit time—hereafter powder rate—fed by a dense phase powder pump in particular depends on the size (volume) of the feed chamber, on the frequency at which coating powder is aspirated into the feed chamber and then discharged from it, on the magnitude of the vacuum, on the time the powder intake valve is open during suction and on the flow impedances in the powder conduits upstream of the dense phase powder pump and especially downstream of it. The flow impedances depend in particular on the length and the inside cross-section of the powder conduits, mostly powder hoses. The compressed conveying air mixes only little with the coating powder which it pushes through the powder outlet valve out of the feed chamber.
Different conditions apply to dilute phase powder pumps using injectors as the powder pump to feed the coating powder. Using a stream of conveying compressed air, a partial vacuum is generated in the injector. This partial vacuum aspirates coating powder into the conveying flow of compressed air. The mixture of powder and conveying compressed air stream moves from the injector to a target site, for instance a bin or a spray tool. The powder rate fed by the injector depends on the rate of conveying compressed air passing through the injector. Powder spraycoating equipment fitted with an injector illustratively is known from U.S. Pat. No. 4,284,032 which is incorporated by reference herein. U.S. Pat. No. 4,357,900 which is incorporated by reference herein discloses powder spraycoating equipment wherein objects to be coated are moved through a cabin wherein they are automatically coated by spray tools driven by sensors, one of such sensors notifying a control unit when an object to be coated is being moved into said cabin in order that the spray tool be activated when said object moves into the coating range of said tool. Another sensor determines the kind of object involved, the electrical signals transmitted by this second sensor determining automatically the powder rate to be deposited on said object. EP 0 412 289 B1 which is incorporated by reference herein discloses an electrostatic powder spraycoating apparatus fitted with an injector and with means keeping constant the total quantity of air fed to the spray tool and consisting of the conveying compressed air plus supplemental air that is added to the stream of powder. EP 0 636 420 A2 discloses powder spraycoating apparatus fitted with a control allowing adjusting the rate of fed powder and—depending on that adjustment and using stored functions—adjusting the rate of conveying compressed air and a rate of supplemental compressed air. Said functions are stored in graphic form.
In the state of the art, the computerized controllers used for injector-fitted powder spraycoating equipment differ from those used for powder spraycoating equipment fitted with dense phase powder pumps.